Zhang Jinglei, Yuan Chengfu, Wu Jianguo, Elsayed Zeinab, Fu Zheng
From the Department of Human and Molecular Genetics, Virginia Commonwealth University Institute of Molecular Genetics, Virginia Commonwealth University Massey Cancer Center, Virginia Commonwealth University School of Medicine, Richmond, Virginia 23298.
From the Department of Human and Molecular Genetics, Virginia Commonwealth University Institute of Molecular Genetics, Virginia Commonwealth University Massey Cancer Center, Virginia Commonwealth University School of Medicine, Richmond, Virginia 23298
J Biol Chem. 2015 Feb 6;290(6):3708-19. doi: 10.1074/jbc.M114.634386. Epub 2014 Dec 22.
Transcription factor Forkhead box protein M1b (FoxM1b) plays an important role during mitotic entry and progression. Our previous studies identified polo-like kinase 1 (PLK1) as a major regulator of FoxM1b. During G2/M transition, PLK1 directly interacts with and phosphorylates FoxM1b, resulting in full activation of the transactivation capacity of FoxM1b. Such a vital regulatory mechanism is essential for timely mitotic entry and progression. However, the molecular mechanism by which PLK1-mediated phosphorylation enhances the transcriptional activity of FoxM1b remains to be determined. We demonstrate that FoxM1b can be SUMOylated in vitro and in vivo, preferentially by SUMO-1. SUMOylation of FoxM1b was found to occur at multiple sites, leading to suppression of FoxM1b transcriptional activity. Such a posttranslational modification of FoxM1b was antagonized by PLK1-mediated phosphorylation. By immunofluorescence staining and subcellular fractionation, we demonstrate that SUMO conjugation promotes cytosolic translocation of FoxM1b. Moreover, SUMO modification of FoxM1b facilitates the ubiquitin-mediated proteasomal degradation of FoxM1b. PLK1-mediated phosphorylation of FoxM1b abrogates the inhibitory effect on FoxM1b by SUMO modification, thereby promoting its nuclear translocation and preventing its proteolytic degradation in the cytoplasm. Such an antagonistic regulatory mechanism is essential for the mitotic function of FoxM1b, ensuring timely mitotic entry and progression. Taken together, our studies have revealed a working mechanism by which PLK1 positively regulates the activity and level of FoxM1b, which would greatly facilitate therapeutic interventions that focus on targeting the PLK1-mediated and/or FoxM1-mediated signaling network.
转录因子叉头框蛋白M1b（FoxM1b）在有丝分裂进入和进程中发挥重要作用。我们之前的研究确定polo样激酶1（PLK1）是FoxM1b的主要调节因子。在G2/M期转换过程中，PLK1直接与FoxM1b相互作用并使其磷酸化，从而导致FoxM1b的反式激活能力完全激活。这种重要的调节机制对于及时进入有丝分裂和进程至关重要。然而，PLK1介导的磷酸化增强FoxM1b转录活性的分子机制仍有待确定。我们证明FoxM1b在体外和体内均可被SUMO化，优先被SUMO-1修饰。发现FoxM1b的SUMO化发生在多个位点，导致FoxM1b转录活性受到抑制。FoxM1b的这种翻译后修饰被PLK1介导的磷酸化所拮抗。通过免疫荧光染色和亚细胞分级分离，我们证明SUMO缀合促进FoxM1b的胞质易位。此外，FoxM1b的SUMO修饰促进了泛素介导的FoxM1b蛋白酶体降解。PLK1介导的FoxM1b磷酸化消除了SUMO修饰对FoxM1b的抑制作用，从而促进其核易位并防止其在细胞质中的蛋白水解降解。这种拮抗调节机制对于FoxM1b的有丝分裂功能至关重要，确保及时进入有丝分裂和进程。综上所述，我们的研究揭示了一种工作机制，通过该机制PLK1正向调节FoxM1b的活性和水平，这将极大地促进针对PLK1介导和/或FoxM1介导的信号网络的治疗干预。
